This invention relates to an electrophotographic image forming apparatus.
An electrophotographic color image forming method used, for example, by a copying machine or printer forms a visible image by forming a toner image on a photoreceptor or belt-type transfer member by a toner image forming unit, transferring the toner image to a transfer material by an image transferring device, and fixing the transfer material. The toner left unused on the belt-type transfer member is removed by a cleaning device.
One of toner cleaning devices uses a bias roller such as a conductive brush roller to remove residual toner electrostatically. Usually, however, toner particles left on the belt-type transfer member have both positive and negative charges even when toner particles in the toner image forming unit, for example, in a developer are charged negatively. This is because the toner particles are charged oppositely to the charge polarity of the toner particles in the developer by the transferring electric field formed in the transferring unit. Therefore, it is impossible for such a cleaning device to remove the positively- and negatively-charged residual toner particles by a single brush roller.
To solve such a problem, a cleaning device has been proposed which has, for example, two brush rollers one of which has a positive cleaning voltage and the other has a negative cleaning voltage (for example, in Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)).
However, there is the possibility that such a cleaning device cannot completely remove toner particles to form toner patches for detection of image density on a belt-type transfer member in order to control the density and gray scale of a visible image.
This is because the toner patch formed on the belt-type transfer member remains non-transferred on the belt-type transfer member when the transferring unit is not working and because the toner is too much to be removed by the above cleaning unit.
Similarly, when a transfer material jams, the transferring unit stops and a lot of non-transferred toner particles remain on the belt-type transfer member. This causes a similar problem.
To overcome the abovementioned problems, there has been proposed another cleaning unit, which applies a specific and large cleaning bias voltage when a large amount of non-transferred toner happens to remain (for instance, set forth in Tokkai 2000-04079 (Japanese Non-Examined Patent Publication)).
However, since it is necessary for the above-mentioned cleaning unit to set the cleaning bias voltage at such a value that is appropriate for the most severe condition for removing the large amount of non-transferred toner, there have been problems that the abovementioned cleaning unit should have a high-power outputting capability with a power source having a large capacity, and therefore, a danger of electric current leak becomes a high-risk factor. Specifically, in a configuration in which a pair of plus and minus electrodes create an electric field, the cleaning bias voltage becomes very high when cleaning non-transferred toner.
Another cleaning device has been proposed which contains a bias voltage applying device and two cleaning brushes between which voltages of an identical polarity are changed (for example, in Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication)).
[Patent Documents 1]                Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication)        
[Patent Documents 2]                Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)        
[Patent Documents 3]                Tokkai 2000-04079 (Japanese Non-Examined Patent Publication)        
Further, the method of changing the same voltage between the cleaning brushes to the bias voltage applying unit and two cleaning brushes (for instance, set forth Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)).
[Patent Documents 4]                Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication)        
Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication) disclose an image forming apparatus which forms, on an intermediate transfer member, a patch image to control the image density and a patch image to correct timing of forming an image of each color (Y, M, C, and K) to form a color image. However, in such an image forming apparatus, an image formed on the intermediate transfer member must be cleaned after the image density is controlled or image timing is corrected. Further, if a transfer material jams before a toner image is transferred from the intermediate transfer member to the transfer material, a lot of toner (if any) on the intermediate transfer member cannot be removed by a single cleaning process and some toner may be left on the intermediate transfer member after the cleaning process. This toner left on the intermediate transfer member will cause image problems such as color mingling in image formation, stains on the back side of the transfer material, and insufficient image density control.
Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) also discloses a cleaning method which contains a bias voltage applying device and two cleaning brushes between which voltages of the same polarity are changed. However, the additional bias voltage applying device makes the configuration complicated.
To solve the above problem, it may be possible to conceive a method of providing two cleaning modes and changing polarities of the brush rollers to clean toner left non-transferred on the intermediate transfer member. The first cleaning mode forms an image according to normal image, transfers the toner image to a transfer material, and reverses the polarity of voltages applied to the two brush rollers to remove toner particles left on the intermediate transfer member.
The second cleaning mode applies voltages of a positive polarity to the two brush rollers to remove toner particles left on the intermediate transfer member since the non-transferred toner particles are charged negatively (which is the regular polarity).
However, we found that this cleaning method cannot be free from causing image problems and insufficient image density control that generated by the cleaning rollers of Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication) since toner particles of a positive polarity brushed out by the brush rollers whose polarity was changed from negative to positive stuck to the intermediate transfer member and remained after the cleaning process.